(1) Field of the Invention
The present invention relates to an image forming apparatus wherein the toner image formed on the photoreceptor is transferred to a printing medium, and more particularly relates to a color image forming apparatus wherein a plurality of color separated images are successively superimposed onto a printing medium to thereby reproduce and output a color image.
(2) Description of the Prior Art
In a color image forming apparatus, for example, a digital color copier, the image of an original captured by the scanner is subjected to predetermined image processings, and then output as a color copy from the printer unit.
For example, Japanese Patent Publication Hei 1 No.45,632, discloses a color image forming apparatus with which the image of a color original is separated into color components and read by a color CCD and the thus captured color-separated images of the color original are stored into the memory, then the data is sequentially read out therefrom to reproduce a color image through the recording unit.
In accordance with the color image forming apparatus written in this disclosure, color-separated images of the color original picked up by the color CCD are temporarily stored, separated by individual colors in buffer memory. Then, the thus stored color-separated sets of image information are successively read from the buffer memory and are input to the semiconductor lasers, which, based on the color-separated sets of image information, reproduce toner images of different colors on respective photoreceptors. Finally, the images of the different colors are superimposed on the printing medium on the transfer drum, thus producing a color image.
This method, however, has suffered from a problem in that producing a color image needs too much time because a color image is reproduced by superimposing a multiple number of mono-color images onto a printing medium (a sheet of paper) supported on the transfer drum.
To deal with this, a recent disclosure in Japanese Patent Application Laid-Open Hei 4 No. 341,873 reveals a color recording apparatus in which recording units for recording individual mono-color images are arranged in parallel with each other.
This method can achieve high speed recording because image forming is controlled so that individual recording units for individual colors of image data are provided in parallel with each other and the image is progressively recorded as the paper is conveyed from one recording unit to another.
Further, there are various transfer methods whereby the toner image of a different color formed on the photoreceptor in each recording unit is transferred to the printing medium.
The most typical conventional method has been use of corona discharge wire, but this method causes ozone generation, which is problematical given the recent environmental consciousness, or gives rise to the manufacturing problem that compacting the whole machine body becomes difficult.
Therefore, recent tendency is to adopt the transfer method using a transfer roller, as also disclosed in the above publication, in which the transfer roller biased at a predetermined voltage is used to press a printing medium through a transfer belt from its inner side against the photoreceptor having an image formed thereon so as to transfer the image on the photoreceptor to the printing medium.
This roller transfer method, however, has a problem in that foreign substances such as dirt, toner etc. attaching to the transfer roller surface and/or the inner side of the transfer belt affect the transfer efficiency, causing the printing medium to be ruffled, vibrated and/or conveyed unevenly. Such defects are fatal in a color process in which different toner images need to be precisely superimposed one over another on the printing medium, resulting in degradation of the image.
Japanese Patent Application Laid-Open Hei 1 No. 177,578 discloses a cleaning device for cleaning the surface of the transfer belt supporting rollers which support and tension the transfer belt. However, this method may still cause the same problem because the roller surface is not stained directly but is stained by foreign substances temporarily adhering on the inner side of the transfer belt and transferring to the support rollers during the rotational movement of the transfer belt.